Variables

A box with a name

• A variable is a named box that holds a value.
• You put a value in the box, then use the name later.
• The value can be a number or a string.

Make a variable

• Write name = value to store a value.
• The name is on the left. The value is on the right.
• = means "store this value", not "is equal to".

city = "Paris"
print(city)

Naming rules

• Use letters, digits, and the underscore _.
• A name cannot start with a digit and cannot have spaces.
• Names are case-sensitive: score and Score are different.
• Pick clear names like age or total_score.

Change a variable

• You can store a new value at any time.
• The new value replaces the old one.
• The name still points to one value — the latest one.

count = 1
count = 2
print(count)

Constants

• A constant is a value that should not change.
• Python has no special keyword for it.
• By convention, we write constant names in UPPERCASE.

PI = 3.14159
print(PI)

Common mistakes

• Use = to store a value: the name goes on the left, the value on the right.
• Use a variable only after you have given it a value, or you get a NameError.
• Names cannot start with a number or contain spaces.

Now you try

• Use a variable for each task below.
• Press Check answer to test your code.